1. Field of the Invention
The present invention relates to a solid oxide fuel cell (SOFC).
2. Description of Related Art
An SOFC includes a porous fuel electrode for allowing reaction of a fuel gas to proceed; a porous air electrode for allowing reaction of an oxygen-containing gas to proceed; and a dense solid electrolyte membrane provided between the fuel electrode and the air electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-226789). In the SOFC, at a high temperature (e.g., 500 to 1,000° C.), when a fuel gas (e.g., hydrogen gas) is supplied to the fuel electrode, and an oxygen-containing gas (e.g., air) is supplied to the air electrode, chemical reactions shown in the following formulas (1) and (2) occur. Thus, a potential difference is generated between the fuel electrode and the air electrode. This potential difference is based on the oxygen ion conductivity of the solid electrolyte.(½).O2+2e−→O2−(at air electrode)  (1)H2+O2−→H2O+2e−(at fuel electrode)  (2)
The following description will be focused on the reaction at the fuel electrode (the aforementioned formula (2)). The porous fuel electrode is formed of, for example, Ni (nickel) and YSZ (yttria-stabilized zirconia). In this fuel electrode, Ni serves as a “catalyst for dissociating hydrogen molecules (H2) into hydrogen ions (2H+),” and also as a “substance for conducting the generated electrons (2e−).” YSZ serves as a “substance for conducting oxygen ions (O2−).” Pores serve as “passages for hydrogen molecules (H2) and the generated water (H2O).” That is, the reaction at the fuel electrode greatly depends on the states of Ni grains, YSZ grains, and pores present in the fuel electrode (particularly in a region of the below-described active layer of the fuel electrode in the vicinity of the interface between the active layer and the solid electrolyte membrane). Therefore, conceivably, the reaction resistance of the fuel electrode can be reduced by controlling the states (e.g., grain sizes) of Ni grains, YSZ grains, and pores present in the fuel electrode.
Hitherto, when a cross section of an SOFC cell sample in a thickness direction (layer-stacking direction) is observed under a scanning electron microscope (SEM observation), “Ni grains or YSZ grains” are easy to distinguish from pores, but Ni grains are very difficult to distinguish from YSZ grains. This is attributed to the high light-dark contrast (more specifically, light-dark contrast in a SEM backscattered electron image) between “Ni grains or YSZ grains” and pores, and the low light-dark contrast between Ni grains and YSZ grains.
In contrast, in recent years, there has been reported an SEM observation technique capable of distinguishing Ni grains from YSZ grains; i.e., capable of distinguishing Ni gains, YSZ grains, and pores from one another. Such a “new SEM observation technique” is described in detail in Solid State Ionics 178 (2008) 1984.
The present inventors have observed cross sections of various fuel electrodes through this “new SEM observation technique,” and have focused particularly on a “region of a fuel electrode in the vicinity of the interface between the fuel electrode and a solid electrolyte membrane,” which region is considered to greatly affect the aforementioned reaction at the fuel electrode. As a result, the present inventors have found that a combination of the sizes of Ni grains, YSZ grains, and pores in the aforementioned region greatly affects the reaction resistance of the fuel electrode. Also, the present inventors have found that an appropriate combination of the sizes of Ni grains, YSZ grains, and pores in the aforementioned region is required for reducing the reaction resistance of the fuel electrode.